The present invention relates to the braking of vehicles and in particular to improvements in the distribution of braking torque between the front and rear wheels of a vehicle under heavy braking.
It is known to control electronically the level of braking torque applied to each wheel of a vehicle to achieve various desirable functions or braking characteristics. Generally these control strategies are based on a number of fundamental concepts. The degree of slip of a particular wheel, i.e. the difference between the wheel speed of that wheel and the actual speed of the vehicle, increases with the level of braking applied to the wheel. High levels of slip produce higher levels of braking, up to a point. Thereafter when slip becomes too high the wheel starts to lock up, the available traction, and hence braking, falls off, and the driver loses control of the vehicle.
ABS systems monitor the wheel speeds of the vehicle during braking and detect if one or more of the wheels starts to lock up, in which case braking torque to the locking wheel is reduced to avoid full wheel locking. Electronic brake distribution (EBD) systems are also known which control the distribution of braking torque between the front and rear wheels of a vehicle. These EBD systems allow for the fact that, under braking, a large proportion of the vehicle""s weight is shifted to the front wheels, and therefore much more torque can be applied to them before they lock up than can be applied to the rear wheels. EBD systems therefore control the level of braking of the front and rear brakes so that the slip of the rear wheels is less than the slip of the front wheels. This maximizes the total level of braking available whilst minimizing the risk of instability due to slipping of the rear wheels.
It can be a problem with known EBD systems that under heavy braking the bias of the braking towards the front brakes, combined with fade of the front brakes due to their increasing temperature, can lead to a slight loss of effective braking. Various proposals have been made to address this problem, for example by redistributing brake torque in a manner which is dependent on brake temperature so as to compensate for fade. The present invention aims provide further improvements to such systems.
Accordingly the present invention provides a braking system for a vehicle having a front wheel and a rear wheel, the system comprising front braking means, which includes a friction surface, for applying a braking torque to the front wheel, rear braking means for applying a braking torque to the rear wheel, brake torque proportioning means for controlling the ratio of braking torque at the rear wheel to the braking torque at the front wheel, brake temperature determining means for determining the temperature of said friction surface of the front braking means, cornering detection means for detecting cornering of the vehicle, and control means arranged to increase the proportion of braking torque applied to the rear wheel in response to an increase in said temperature when the vehicle is travelling straight ahead, but to at least partially override said increase of the proportion of braking torque applied to the rear wheel if cornering of the vehicle is detected.
Preferably the control means is arranged to control the amount of slip of the rear wheels, and to increase the proportion of braking torque applied to the rear wheels by increasing the level of slip of the rear wheels.
Preferably the brake temperature determining means is arranged to estimate the brake temperature from at least one parameter indicative of the operation of the vehicle. The parameters may include the speed of at least one wheel of the vehicle and an indicator of the braking torque applied to at least one wheel of the vehicle. Alternatively the temperature may be directly measured.
The present invention further provides a braking system for a vehicle having a front wheel and a rear wheel, the system comprising a front brake, which includes a friction surface, for applying a braking torque to the front wheel, rear brake for applying a braking torque to the rear wheel, a modulator for controlling the ratio of braking torque at the rear wheel to the braking torque at the front wheel, brake temperature sensor for sensing the temperature of said friction surface of the front brake, cornering sensor for detecting cornering of the vehicle, and a controller arranged to increase the proportion of braking torque applied to the rear wheel in response to an increase in said temperature when the vehicle is travelling straight ahead, but to at least partially override said increase of the proportion of braking torque applied to the rear wheel if cornering of the vehicle is detected.